Definitely a cnidarian.

If you want a correct identiication, post the image on

http://doris.ffessm.fr/

It could be some kind of Zoanthus or related genera.

I agree with Vinicus. I'm fairly certain that this is a species of Zoanthus (soft coral), likely a Violet zoanthid (Zoanthus sansibaricus).

I agree with Jeff and Vinicius, Zoanthus!

Looks like a mat zoanthid. Zoanthus pulchellus or similar - knowing the region or ocean basin would be helpful for a proper ID!

Just out of curiosity: have conducted a google image search? I was just wondering how good an image search would be for identifying/ finding species.

Inka, I tried google's "search by image" to search for visually similar images, and uploaded the image at various crops. Uncropped one yield images of brown algae like Fucus; while cropped one zoomed to the organism yielded plants of similar color! So search by image is quite useless for species identification, but ResearchGate is, and thanks a million Vinicius and others! It seems like a species of Zoanthus.

Where have you seen them?

It's hard to tell from the picture, but they could also be sea anemones. 

Certainly a Cnidarian... However through a photo and that resolution is very difficult to identify the species... 

Yes, it is a clonally reproducing anemone like those common on wave exposed shores of Tatosh Island USA that Ken Sebans worked on in the late 70s.

Cnidarian. I agree with Mr. Jérôme Mallefet. I also found the similar species at our coastline.

Yes. This anemone is in its contracted state. I think it is this species: 

https://www.youtube.com/watch?v=sUYK9nlJ4kI

Anthopleura nigrescens occurs in coastal waters around India.

Yes, it is Cnidarian  found in coastal region also.

Anthopleura sp. because it mostly associated with Crassostera sp.